Porous polymer networks and ion-exchange media and metal-polymer composites made therefrom

ABSTRACT

Porous polymeric networks and composite materials comprising metal nanoparticles distributed in the polymeric networks are provided. Also provided are methods for using the polymeric networks and the composite materials in liquid- and vapor-phase waste remediation applications. The porous polymeric networks, are highly porous, three-dimensional structures characterized by high surface areas. The polymeric networks comprise polymers polymerized from aldehydes and phenolic molecules.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. non-provisional patentapplication Ser. No. 13/888,705 that was filed on May 7, 2013, whichclaims priority to U.S. provisional patent application No. 61/643,525that was filed May 7, 2012, the entire contents of which are herebyincorporated by reference.

REFERENCE TO GOVERNMENT RIGHTS

This invention was made with government support underDE-FG36-08GO18137/A001 awarded by Department of Energy. The governmenthas certain rights in the invention.

BACKGROUND

Porous polymers are a rapidly expanding category of materials. Thesepolymers are characterized by a three dimensional network incorporatingaromatic rings. They have been synthesized through a variety of organicreactions that produce stable linkages between rigid monomers and have aconnectivity of greater than two. They are highly porous and, as such,able to provide high surface area materials. Drawbacks to the use ofthese materials are the high cost of the starting materials and thenon-scalability of the polymerization reactions. Proposed applicationsof porous polymers include gas storage and separations.

SUMMARY

Porous, three-dimensional, aromatic polymeric networks comprising anorganic polymer polymerized from aldehyde molecules and phenolicmolecules are provided. In some embodiments, the walls of the polymericnetwork that define the pores in the polymeric network arefunctionalized with ion-exchangeable cations.

The polymeric networks comprising the ion-exchangeable cations can beused in methods for remediating hazardous materials, such as heavymetals, in the liquid or vapor-phase. These methods comprise exposingthe polymeric networks to a sample comprising metal ions capable ofundergoing ion-exchange with the ion-exchangeable cations, whereby saidion exchange occurs; and subsequently removing the polymeric networkfrom the sample.

Also provided are composite materials that incorporate the porouspolymeric networks. These composite materials comprise: (a) a porous,three-dimensional, aromatic polymeric network comprising an organicpolymer polymerized from aldehyde molecules and phenolic molecules; and(b) metal nanoparticles distributed within the polymeric network.

The composite materials can be used in methods for remediating hazardousmaterials from samples, including vapor-phase samples. These methodscomprise exposing the composite material to a vapor-phase samplecomprising an unwanted element or molecule, such as iodine or Hg,whereby the unwanted element or molecules is adsorbed in the pores ofthe polymeric network; and subsequently removing the composite materialfrom the sample.

Other principal features and advantages of the invention will becomeapparent to those skilled in the art upon review of the followingdrawings, the detailed description, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative embodiments of the invention will hereafter be describedwith reference to the accompanying drawings, wherein like numeralsdenote like elements.

FIG. 1. The chemical structures for various phenolic molecules.

FIG. 2. The chemical structures for various aldehyde molecules.

FIG. 3. Solid State ¹³C Cross Polarization Magic Angle Spinning (CPMAS)NMR of Mon-POF. The bullets correspond to the sidebands of the 128 ppmpeak.

FIG. 4. Thermogravimetric analysis of Mon-POF under N₂ and air.

FIG. 5. Images of as prepared (A) and dried (B) Mon-POF and thecross-section of the dried Mon-POF (C).

FIG. 6. SEM image of Mon-POF.

FIG. 7. (A) N₂ adsorption-desorption isotherm and (B) non-local densityfunctional theory (NLDFT) pore size distribution of Mon-POF.

FIG. 8. The evolution of Mon-POFs porosity with concentration ofmonomers in dioxane and amount of catalyst.

FIG. 9. Solid state Electron Paramagnetic Resonance (EPR) spectra ofMon-POF.

FIG. 10. The solid state electronic absorption spectra of Mon-POFs(lower curve=Mon-POFcc; middle curve=Mon-POFc; upper curve=Mon-POF).

FIG. 11. (A) (B) TEM images of Ag nanoparticles (dark spots) dispersedin Mon-POF. (C) (D) TEM images of Ag nanoparticles (bright spots) inMon-POF.

FIG. 12. (A) The x-ray diffraction (XRD) pattern of Ag@Mon-POF. Thebullets correspond to Ag diffraction peaks. (B) N₂ ads.-des. isotherm ofAg@Mon-POF.

FIG. 13. (A) The XRD pattern and (B) thermogravimetric analysis ofAg@Mon-POF after iodine capture.

FIG. 14. Solid State ¹³C CPMAS NMR of Mac-POF-1. The bullets correspondto the sidebands of the 128 ppm peak.

FIG. 15. N₂ adsorption-desorption isotherm of Mac-POF-1.

FIG. 16. Solid state EPR spectra of Mac-POF-1.

FIG. 17. TEM image of Ag nanoparticles (bright spots) in Mac-POF-1.

FIG. 18. (A) The XRD pattern of Ag@Mac-POF-1. The bullets correspond toAg diffraction peaks. (B) N₂ ads.-des. isotherm of Ag@Mac-POF-1.

DETAILED DESCRIPTION

Porous polymeric networks and composite materials comprising metalnanoparticles distributed in the polymeric networks are provided. Alsoprovided are methods for using the polymeric networks and the compositematerials in liquid- and vapor-phase remediation applications.

The porous polymeric networks, which are also referred to as polymericorganic frameworks (POFs), are highly porous, three-dimensionalstructures characterized by high surface areas. The polymeric networkscomprise polymers polymerized from aldehydes and phenolic molecules. Insome embodiments, the aldehydes comprise non-phenolic aromatic rings,such as six-membered aromatic rings. Thus, the resulting polymernetworks comprise phenolic groups that are linked to one another throughlinkages comprising one or more non-phenolic aromatic rings.

The phenolic molecules have reactive sites suitable electrophilicaromatic substitution, and during polymerization they can be linkedthrough a formaldehyde carbon, followed by elimination of a molecule ofH₂O. The phenolic molecules can comprise a single aromatic ring, or caninclude two or more fused or unfused aromatic rings. Examples ofphenolic molecules that can be used in the polymerization of the POFsinclude those comprising 2 or more hydroxy groups, such ashydroxynaphthalenes (e.g., dihydroxynaphthalenes, such as1,5-dihydroxynaphthalene) and phloroglucinol (1,3,5-trihydroxybenzene).The structures of 1,5-dihydroxynaphthalene and phloroglucinol are shownin FIG. 1, panels (H) and (I), respectively. The structures of othersuitable electrophilic phenolic molecules are shown in FIG. 1, panels(A)-(G). As illustrated in this figure, the phenolic group can befunctionalized with an —SH group or an —NH₂ group.

The aldehydes with which the phenolic molecules react to form theorganic polymer include dialdehydes, trialdehydes, and higherpolyaldehydes. Like the phenolic molecules, the aldehydes may bearomatic molecules (e.g., benzaldehydes) with a single aromatic ring orwith two or more (e.g., three or more, four or more, etc.) fused orunfused aromatic rings. Terephthalaldehyde is one example of an aldehydethat can be reacted with the phenolic molecules. The structure ofterephthalaldehyde is shown in FIG. 2, panel (A). The structures ofother suitable aldehydes are shown in FIG. 2, panels (B)-(I).

By way of illustration only, POFs that can be used to form the porouspolymeric networks include POFs comprising the polymerization product ofphloroglucinol and a benzaldehyde derivative. Specific examples of theseare discussed in Katsoulidis et al., Chem. Mater. 2011, 23, 1818-1824,the entire disclosure of which is incorporated herein by reference.Other examples include POFs comprising the polymerization product of1,5-dihydroxynaphthalene and terephthalaldehyde, which is described ingreater detail in the Example I. The macroporous polymer derived fromthe acid catalyzed polymerization between phloroglucinol andterephthalaldehyde is described in greater detail in Example II.

Because the polymeric networks are highly porous, they are able toprovide high-surface area materials. Some embodiments of the polymericnetworks have BET surface areas of at least 500 m²/g. This includespolymeric networks having BET surface areas of at least 900 m²/g, atleast 1000 m²/g and at least 1200 m²/g. The pores of the polymericnetworks are of different sizes and they are extended in the threecategories of pores (micropores, mesopores, and macropores). Thus, insome embodiments of the polymeric networks, the average pore diameter ofthe distribution of pores is greater than about 500 Å (macropores).However in other embodiments, the average pore diameter of thedistribution of pores is no greater than about 500 Å (mesopores). Thisincludes embodiments in which the average pore diameter of thedistribution of pores is no greater than about 20 Å (micropores).

The walls of the porous polymeric network that define the pores can befunctionalized with various chemical functionalities. For example, theas-formed polymeric networks typically have a high density of hydroxylfunctionalities on the pore walls. For example, in some embodiments, thedensity of hydroxyl functionalities is in the range from 1 to 2 —OHgroups per phenyl ring of the polymer. Alternatively, some or all of thehydroxyl groups can be deprotonated via ion exchange of the hydroxylproton with other cations, such as alkali metal or alkaline earth metalcations. This can be accomplished, for example, by exposing the hydroxylfunctionalized polymeric network to the other cations under basicconditions.

The resulting cation-exchanged polymeric networks can themselves be usedas ion-exchange media in waste remediation applications, such as metalremediation in liquid (e.g., aqueous) and vapor-phase samples. In suchapplications, the ion-exchange medium is exposed to a sample comprisingunwanted metal ions that are capable of undergoing ion exchange with thecations of the ion-exchange medium, under conditions in which said ionexchange occurs. The ion-exchange medium and the unwanted ions can thenbe removed from the sample. Metal ions that can be remediated using thepresent polymeric networks include heavy metals, such as Ag⁺, Au⁺, Hg²⁺,Cu²⁺, Pb²⁺, Cs⁺ and Tl⁺. Other metal ions that can be remediated includeSn ions, Bi ions and Sb ions.

Notably, in some instances, the material that results from theion-exchange between the ion-exchange medium and metal ions forms auseful composite material formed by the reduction of the metal ions bythe organic polymer of the polymeric network. These composites comprisethe porous, three-dimensional, polymeric network comprising the organicpolymer polymerized from aldehyde molecules and phenolic molecules; andmetal nanoparticles distributed within the polymeric network. An exampleof such a composite material comprising silver nanoparticles depositedin a POF is described in detail in the Example below. (As used herein, ananoparticle is a particle having a diameter of no greater than about100 nm. Thus, nanoparticles include particles having diameters of nogreater than about 50, no greater than about 10 and no greater thanabout 5 nm.) The metal loading in these composite materials can besubstantial. For example, in some embodiments, the composite materialscomprise at least 10 percent by weight (wt. %) metal, based on the totalweight of the nanoparticles and the organic polymer of the polymericnetwork. This includes composite materials that comprise at least 15 wt.%, at least 20 wt. % and at least 25 wt. % metal, based on the totalweight of the nanoparticles and the organic polymer of the polymericnetwork.

The composite materials are useful in a wide variety of applicationswhere supported metal nanoparticles have already proven useful. Theseinclude applications in catalysis, optics and anti-microbials. Oneapplication where the composite materials can be particularlyadvantageous is as adsorbents in the vapor-phase remediation ofhazardous wastes, including radioisotopes, such as 129-iodine, that arereleased during the processing of nuclear fuels. In such applications,the composite material is exposed to a vapor-phase sample comprising anunwanted vapor-phase molecule or element, such as iodine or Hg(g),whereby the unwanted element is adsorbed in the pores of the POF. Thecomposite material can then be removed from the sample.

Example I

This example illustrates a polymeric organic framework in the form of amonolithic polymeric organic framework, Mon-POF, prepared fromterephthalaldehyde and 1,5-dihydroxynaphthalene. It exhibits highsurface area, 1230 m² g⁻¹, and low bulk density, 0.15 gcm⁻³. Mon-POFreduced Ag⁺ to Ag nanoparticles forming a nanocomposite material with Agloading of ˜25 wt. %. The Ag loaded monolith captured iodine vapors andfixed them in the form of β-AgI.

Materials.

All reagents and solvents were used as received unless noted otherwise.Terephthalaldehyde, 1,5-dihydroxynaphthalene, silver nitrate, iodine and1,4-dioxane were purchased from Aldrich Chemical Co. tetrahydrofuran(THF), ethanol, HCl and NaOH were purchased from VWR.

Synthesis of Monolith.

In a round bottom flask an amount of 0.320 g (2 mmol) of1,5-dihydroxynaphthalene and 0.268 g (2 mmol) of terephthalaldehyde wereadded in 5 ml of dioxane. The mixture was kept under stirring at 70° C.Thirty minutes later 1 ml of aq. HCl 1M was added. The mixture wasallowed to react for 3 h, transferred to a Teflon-lined autoclave, whichwas purged with N₂ to remove the air, and placed in an oven at 220° C.for 4 d. After cooling at room temperature, a brown piece having theinternal shape of the autoclave was obtained. The monolith was placed ina beaker with THF and stayed undisturbed for 3 days to wash out anyunreacted and oligomeric species. The solvent was decanted and refilledtwice each day. After three days the THF was replaced with ethanol,where the monolith stayed for 2 days. Finally the monolith wassupercritically dried with CO₂ using the Autosamdri 815B instrument ofTousimis. The mass of the dried product was 0.52 g and the yield of thepolymerization corresponded to 94%. Similarly, two more aerogels weresynthesized under the same conditions using 1 mmol of each monomer and0.5 ml of aq. HCl 1M (Mon-POFcc) or 1 mmol of each monomer and 1 ml ofaq. HCl 1M (Mon-POFc).

Preparation of Ag Nanoparticles in Mon-POF.

A piece of Mon-POF, 150 mg, was placed in 50 ml of an aqueous solutionof 1M NaOH to exchange the protons of the —OH groups with Na⁺. After 3 hthe Na⁺ exchanged Mon-POF was collected through filtration and washedwith H₂O. The wet Na⁺ Mon-POF was placed in 50 ml of H₂O (resultingpH=10). In that system, 300 mg of AgNO₃ were added and allowed to reactovernight. The collected monolithic piece was washed extensively withH₂O, soaked in ethanol to exchange the H₂O and dried again withsupercritical CO₂. The final product was called Ag@Mon-POF.

Capture of Iodine.

500 mg of I₂ were transferred in a two neck round bottom flask. On thetop of the flask, fritted glassware was connected where a piece (≈50 mg)of Ag@Mon-POF was placed. Iodine vapors were produced after heating theflask at 70° C. and t were driven upwards with the nitrogen flowconnected to the side neck of the flask.

Characterization Methods.

N₂ adsorption—desorption isotherms were measured at 77 K. Themeasurements were carried out in an ASAP 2020 and in a Tristar 3020porosimeter of Micromeritics. The specific surface area was calculatedaccording to the BET method (0.05<P/P₀<0.25). Total pore volume wasestimated from the adsorbed amount at P/P₀=0.97. Micropore volume wasdetermined from t-plots. (Lowell, S.; Shields, J. E.; Thomas, M. A.;Thommes, M. Characterization of porous solids and powders: surface area,pore size and density; Kluwer Academic Publishers: Dordrecht, 2004 p.130.) NLDFT (cylindrical model) was applied to obtain the pore sizedistribution. The skeletal density of the aerogel was determined withhelium pycnometry using the Accupyc II 1340 of Micromeritics. The bulk(geometrical) density was calculated from the physical dimensions of theaerogel. Solid state NMR spectra were recorded in a Varian 400 ATXspectrometer operating at 100 MHz for ¹³C and 400 for ¹H. The ¹³C CPMASmeasurements carried out at spinning rate of 10 kHz. Two pulse phasemodulation (TPPM) ¹H decoupling was applied during the acquisition. The¹³C were given relative to tetramethylsilane as 0 ppm and calibrated byusing adamantane as a secondary reference. XRD powder patterns werecollected on a CPS 120 Inel diffractometer equipped with CuKα radiation.UV-vis-NIR diffuse reflectance spectra (DRS) were recorded with aShimadzu UV-3101PC spectrophotometer. BaSO₄ powder was used as the 100%reflectance standard. The reflectance data were converted to absorptionaccording to the Kubelka-Munk equation a/S=(1−R)²/2R, where R is thereflectance and a and S are the absorption and scattering coefficient,respectively. Thermogravimetric analysis was performed in a ShimadzuTGA-50 thermal analyzer by heating each sample (≈10 mg) from roomtemperature (˜23° C.) to 600° C. with a ramping rate of 5° C. min⁻¹under nitrogen or air flow. Scanning electron microscopy (SEM) imageswere collected in a Hitachi S-3400N instrument with an acceleratingvoltage of 20 kV. High magnification SEM images were collected on a Leo1525 (Carl Zeiss Microimaging Inc.). Before measurement, the sampleswere sputter coated with gold. TEM investigations were carried out in aJEOL 2100F transmission electron microscope operating at 200 kVaccelerating voltage. The sample was dispersed in ethanol and mounted ona carbon coated copper grid.

Results and Discussion

The polymerization between terephthalaldehyde and1,5-dihydroxynaphthalene is depicted in Scheme 1. Each carbonyl groupreacts with two dihydroxynaphthalene molecules eliminating a watermolecule. Thus, terephthalaldehyde is linked with four molecules of1,5-dihydroxynaphthalene (i). On the other hand 1,5-dihydroxynaphthalenehas four reaction sites, the ortho- and para-positions of each hydroxylgroup, and it reacts with four terephthalaldehydes as well (ii). In thisway an extended and highly cross-linked polymeric framework is created.In the past it has been shown that no catalyst is needed for thesolvothermal polymerization between phloroglucinol andterephthalaldehyde. (Katsoulidis, A. P.; Kanatzidis, M. G. Chem. Mater.2011, 23, 1818-1824.) However, 1,5 dihydroxynaphthalene is lessnucleophilic and less reactive than phloroglucinol, having only onehydroxyl group per aromatic ring, instead of the three ofphloroglucinol. In order to polymerize 1,5-dihydroxynaphthalene withterephthalaldehyde, HCl was used to activate the carbonyl groups.

The formation of the polymer according to the above mentioned reactionwas proven with solid state ¹³C CPMAS NMR. A typical spectrum of Mon-POFis presented in FIG. 3, where the side bands of the big peak are denotedwith bullets. The resonance of the aldehyde carbonyl carbons, 195 ppm,did not appear and a new one emerged at 44 ppm, which is attributed tothe methyne bridge carbons. Reacted ortho and para carbons of1,5-dihydroxynaphthalene exhibited a signal at 116 ppm. The big peak at128 ppm peak was assigned to the aromatic carbons and the resonance at150 ppm corresponds to phenoxy carbons. A shoulder at 145 ppmcorresponds to carbons 1 and 4 of the aldehyde originating ring. Nopeaks were observed in the spectra in the range of 55-75 ppm indicatingthe absence of any adsorbed solvent molecule, dioxane, THF or ethanolfrom the synthesis and washing procedure.

The Mon-POF was completely dry after supercritical drying, as proved bythe TGA curves (FIG. 4) where no mass loss was observed up to 330° C.,either under N₂ or under air. The aerogel was gradually decomposed underN₂ at elevated temperatures, but was considered thermally stable,retaining 70% of its initial mass at 600° C. Under air the aerogel wasoxidized rapidly and the combustion was completed at 530° C.

The untreated sample after the solvothermal synthesis had the internalshape of the autoclave (FIG. 5A) and exhibited a bulk (geometrical)density of 1.27 gcm⁻³. Even though the sample looked dry, it contained asignificant amount of dioxane and its mass, 4.8 g, was much higher thanthe monomers' mass, 0.588 g. After solvent exchange with THF and ethanoland CO₂ supercritcal drying, the shape and the dimensions of themonolith were preserved (FIG. 5B) and the bulk density was decreased to0.15 gcm⁻³. It could be seen from the cross-sectional image that thetexture of the sample was very homogeneous (FIG. 5C). Despite the lowbulk density, Mon-POF was strong enough to support a zirconia ball onthe top of it, which was 250 times heavier than the monolithic piece.Mon-POF was handled easily and it could be cut to smaller pieces with ablade. SEM image (FIG. 6) shows that Mon-POF comprises aggregatednanoparticles 20-40 nm in diameter. The skeletal density of driedMon-POF was measured as 1.37 gcm⁻³ according to helium pycnometry. Theporosity, ε, of Mon-POF, calculated from equation (1) where p_(b) andp_(s) are bulk and skeletal density, respectively, equaled 89%.

$\begin{matrix}{ɛ = {\left( {1 - \frac{p_{b}}{p_{s}}} \right)*100\%}} & (1)\end{matrix}$

A typical N₂ adsorption-desorption isotherm of Mon-POF is presented inFIG. 7A. It corresponds to a type II isotherm, according IUPACclassification, suggesting the predominant macroporous character of themonolithic aerogel. But, at the same time, the high uptake at very lowrelative pressure gave evidence of significant microporosity. Thespecific surface area (BET) was calculated as 1230 m² g⁻¹ and the totalpore volume as 1.46 cm³ g⁻¹ (P/P₀=0.97). The existence of micropores onMon-POF was proven using the t-plot method showed a micropore volume of0.2 cm³ g⁻¹ (Table 1). (Lowell, S.; Shields, J. E.; Thomas, M. A.;Thommes, M. Characterization of porous solids and powders: surface area,pore size and density; Kluwer Academic Publishers: Dordrecht, 2004 p.130.) The pore size distribution calculated applying NLDFT exhibited amaximum at 6 Å psd (FIG. 7B) and some lower peaks in the whole range ofdiameters showed that beyond micropores the distribution of pores isvery broad. The distribution in the range of macropores (>500 Å) cannotbe obtained from N₂ porosimetry.

TABLE 1 Synthesis parameters, porous properties and yield of Mon-POFs.Specific surface Total pore Micropore Concentration - area volume volumeYield Sample Catalyst (m²g⁻¹) (cm³g⁻¹) (cm³g⁻¹) (%) Mon-POFcc 1 mmol/5ml - 946 0.84 0.24 84 0.5 ml HCl Mon-POFc 1 mmol/5 ml - 1117 1.17 0.2290 1 ml HCl Mon-POF 2 mmol/5 ml - 1230 1.46 0.20 94 1 ml HCl

The formation of a gel was strongly influenced by the parameters thataffect the polymerization reaction rate, such as temperature, catalystand concentration of reactants. To investigate the effect of reactionrate on the formation of Mon-POFs various samples were synthesized usinghalf the concentration of starting materials and/or half the amount ofcatalyst, in an aqueous solution HCl 1M, under the same temperatureprofile. Mon-POFcc was synthesized using 1 mmol of monomers per 5 ml ofdioxane, instead of the 2 mmol used for the Mon-POF, and 0.5 ml of HCl1M, instead of the 1 ml used for the Mon-POF. Mon-POFc was synthesizedusing 1 mmol of monomers per 5 ml of dioxane and 1 ml of HCl. From bothsyntheses monolithic aerogels were obtained in lower yield, 84 and 90%for Mon-POFcc and Mon-POFc, respectively, (Table 1) in comparison to 94%of Mon-POF. They exhibited similar molecular structure as Mon-POF, basedon the fact that the ¹³C CPMAS NMR spectra were very similar and the N₂adsorption-desorption isotherms were also of type II.

The porous properties of the monoliths are also listed in Table 1. Thespecific surface areas and the total pore volumes of Mon-POFs followedthe same trend as the yield of the polymerization, which increased withthe reaction rate. On the other hand, the values of micropore volumeshowed the opposite trend, decreasing in moving from Mon-POFcc toMon-POF. The evolution of the porous properties can be explained byconsidering the framework's extension and relaxation. At low reactionrate the fragments of the framework have time to relax and achievebetter packing before growing larger, resulting in materials with highermicroporosity. On the other hand, at higher reaction rates the frameworkgrew rapidly, with less time to pack efficiently, forming larger andmore tortuous polymeric units. This produced materials with higher totalpore volume. The effect of the reaction rate on the porous properties ofmonoliths is represented in FIG. 8.

Mon-POF was characterized with continuous wave (CW) EPR spectroscopy atroom temperature (FIG. 9). This compound was paramagnetic and showed astrong EPR signal, which was 4 mT wide and centered around 340.5 mT (gfactor=2.006). The stabilization of unpaired electrons on the POFs was asimultaneous effect of the polymerization reaction and was observed inall the samples of this family. The existence of unpaired electrons areresponsible for the semiconductor like optical properties as explainedbelow.

Like \previous POFs compounds, Mon-POFs showed semiconductor-likeoptical absorption properties. (Katsoulidis, A. P.; Kanatzidis, M. G.Chem. Mater. 2011, 23, 1818-1824.) The solid state absorption spectra ofMon-POFs are given in FIG. 10. Mon-POF exhibited a band gap of 2.3 eVand a broad absorption feature at >4 eV. The other two samples, Mon-POFcand Mon-POFcc, had band gaps at 2.5 eV and the intensity of absorptionfrom 2 to 4 eV was remarkably weaker than that of the Mon-POF. Thedifferences in optical properties were ascribed to the size of thepolymeric units in each monolith. As mentioned above, the higher thereaction rate, the greater the size of the polymeric unit and greaterthe delocalization of electrons within the framework. Thus, Mon-POFprepared at higher reaction rate exhibited the lowest band gap and thehighest optical absorption. The molecular structure of Mon-POF, asdepicted in Scheme 1, is not highly conjugated, however, the existenceof unpaired electrons, which are probably localized on the methinecarbon promoted the conjugation and the optical absorption properties(Scheme 2).

The stability of Mon-POF in water was tested under acidic and basicconditions. Two pieces of Mon-POF were soaked for 24 h in aqueoussolutions of HCl 1M (pH=0) and NaOH 1M (pH=14), respectively. Neither ofthe monoliths dissolved but remained as a single piece. Their ¹³C CPMASNMR spectra after the acid and base treatment were the same incomparison to raw Mon-POF. After the base treatment, there was oneadditional peak for Mon-POF (NaOH), at 165 ppm, which was assigned tophenoxy carbons with protons exchanged with Na⁺ cations. A similarphenomenon of partial exchange of protons with Na⁺ was observed inphloroglucinol POFs as well. (Katsoulidis, A. P.; Kanatzidis, M. G.Chem. Mater. 2011, 23, 1818-1824.) N₂ adsorption-desorption isothermsrevealed that both monoliths retained more than 75% of their surfacearea. Mon-POF (HCl) exhibited a surface area 1059 m² g⁻¹ and Mon-POF(NaOH) exhibited a surface area 929 m² g⁻¹. The porosity of Mon-POFdecreased very slowly with time and no precautions were needed for themonolith's storage. The N₂ adsorption-desorption isotherms were measuredagain in 3 and 6 month intervals after synthesis and the specificsurface areas decreased to 1049 and 914 m² g⁻¹, respectively.

Silver Deposition:

The functionalization of Mon-POF with Na⁺ ion-exchange prompted theinvestigation of some basic ion-exchange properties with metals such asAg⁺. The materials did indeed lose Na and picked up Ag but,surprisingly, the Ag was reduced to metal nanoparticles. Clearly, theMon-POF has reductive properties and is capable of reducing AgNO₃,probably after ion-exchange as the silver ions enter the material. TEMimages of Ag@Mon-POF (FIG. 11) show spherical Ag nanoparticles of 5-10nm in diameter well dispersed in the polymeric framework. The XRDpattern of Ag@Mon-POF (FIG. 12A) shows broad peaks that correspond tonanocrystalline cubic Ag (pdf 04-0783, ICDD) with an average crystallitesize of ˜25 Å according to the Scherrer equation. The monolith, afterthe reaction with AgNO₃, was soaked in ethanol and dried withsupercritical CO₂. The loading of Ag to Mon-POF was significant at ˜25wt. %, as estimated from thermogravimetric analysis. Interestingly theTGA also showed that, in air, the combustion of Ag@Mon-POF was catalyzedby the Ag nanoparticles and occurred in a temperature range from 230° C.to 250° C., in comparison to pristine Mon-POF, which burned out in therange from 300° C. to 520° C. (FIG. 4). The residue of Ag@Mon-POF's TGAwas analyzed with XRD, presenting sharp diffraction peaks of elementalAg. Ag@Mon-POF exhibited an N₂ adsorption-desorption isotherm (FIG. 12B)of type II as the pristine monolith and a specific surface area of 690m² g⁻¹, revealing that the deposited Ag nanoparticles did not block thepores of the monolith.

Ag@Mon-POF composite material combined the properties of low densityporous substrate with metallic nanoparticles. Silver nanoparticles havebeen widely exploited in several fields like catalysis, optics,antimicrobials, biosensing, and SERS and they have been stabilized onmacroporous polymeric substrates through the reduction of AgNO₃ fromNaBH₄ or hydrazine at maximum content ˜7% wt. In the present case AgNO₃was reduced without any additional reductant and the reductiveproperties of Na⁺ Mon-POF is attributed to the phenolic OH groups whatcan be correlated to the reducing activity of natural polyphenols, whichare well known for their antioxidant properties.

Iodine Capture:

With the Ag@Mon-POF at hand a study was conducted as to whether thesilver laden material could capture iodine vapor and stabilize it in theform of AgI, eq. (2). This reaction is relevant for the capture andstorage of radioisotopes released during reprocessing of spent nuclearfuel—particularly 129-iodine.

Ag@Mon-POF+½I₂ AgI@Mon-POF  (2)

The XRD pattern of Ag@Mon-POF after exposure in iodine vapors for 2 hrsis shown in FIG. 13A where the Bragg peaks correspond to hexagonal β-AgI(pdf 09-0374, ICDD) and those of the metallic Ag are absent. The atomicratio of Ag:I in the material after iodine treatment according to EDSwas 2:3, showing excess of I compared to Ag, indicating that in additionto reaction with Ag, iodine vapors were also physically adsorbed in thepores of the monolith. This was verified from the TGA graph (FIG. 13B)of the sample after iodine capture, where a broad mass loss step wasobserved from 90 to 240° C., (I₂ bop.=184° C.). In addition to the highAg loading, 25 wt. %, the effective capture and storage of I₂ onAg@Mon-POF was facilitated by the high dispersion and the small size ofthe Ag nanoparticles, as well as the macroporous structure of themonolith that allowed iodine vapors to diffuse readily and access themetallic phase.

The continued interest for alternative waste forms for ¹²⁹I usingmaterials that can provide higher waste loadings makes the Mon-POFpresented here of significant interest. To capture iodine, aerogels,silver-loaded zeolites and more recently chalcogels and MOFs have beenstudied for confinement of iodine radioactive wastes in recent years andare under investigation as waste forms for ¹²⁹I. Mon-POFs appear to bean attractive alternative to these systems bringing special advantagessuch as high loadings, extreme pH stability and mechanical robustness.

Example II

This example illustrates a macroporous polymeric organic framework,Mac-POF-1, prepared from the acid catalyzed reaction ofterephthalaldehyde and phloroglucinol. It exhibits high surface area,1019 m² g⁻¹. Mac-POF-1 reduced Ag⁺ to Ag nanoparticles forming ananocomposite material with an Ag loading of ˜20 wt. %.

Materials.

All reagents and solvents were used as received unless noted otherwise.Terephthalaldehyde, phloroglucinol, silver nitrate and 1,4-dioxane werepurchased from Aldrich Chemical Co. THF, ethanol, HCl and NaOH werepurchased from VWR.

Synthesis of Mac-POF-1.

In a round bottom flask amounts of 0.504 g (4 mmol) of phloroglucinoland 0.402 g (3 mmol) of terephthalaldehyde were added in 10 ml ofdioxane. The mixture was kept under stirring at 70° C. and 30 min laterthree drops of aq. HCl 1M was added. In the next two min the wholemixture was solidified. The solid was transferred in a Teflon linedautoclave and purged with N₂. The autoclave was heated at 220° C. for 2days. After cooling at room temperature, a dark red solid was obtainedand washed with THF. The solid was dried in vacuum oven at 50° C.overnight. The yield was 95%.

Preparation of Ag Nanoparticles in Mac-POF-1.

100 mg of Mac-POF-1 were mixed with 50 ml of an aqueous solution of 1MNaOH. After 2 h the POFs were collected with filtration. TheNaOH-treated POF was mixed with 50 ml of H₂O, resulting in a pH=10. Inthat mixture 200 mg AgNO₃ were added and allowed to react overnight. Theproduct was collected with filtration.

Characterization Methods.

N₂ adsorption—desorption isotherms were measured at 77 K. Themeasurements were carried out in an ASAP 2020 and in a Tristar 3020porosimeter of Micromeritics. The specific surface area was calculatedaccording to the BET method (0.05<P/P₀<0.25). Total pore volume wasestimated from the adsorbed amount at P/P₀=0.97. Micropore volume wasdetermined from t-plots. (Lowell, S.; Shields, J. E.; Thomas, M. A.;Thommes, M. Characterization of porous solids and powders: surface area,pore size and density; Kluwer Academic Publishers: Dordrecht, 2004 p.130.) NLDFT (cylindrical model) was applied to obtain the pore sizedistribution. Solid state NMR spectra were recorded in a Varian 400 ATXspectrometer operating 100 MHz for ¹³C and 400 for ¹H. The ¹³C CPMASmeasurements were carried out at a spinning rate of 10 kHz. Two pulsephase modulation (TPPM)¹H decoupling was applied during acquisition. The¹³C were given relative to tetramethylsilane as 0 ppm and calibrated byusing adamantane as a secondary reference. XRD powder patterns werecollected on a CPS 120 Inel difractometer equipped with CuKα radiation.UV-vis-NIR diffuse reflectance spectra (DRS) were recorded with aShimadzu UV-3101PC spectrophotometer. BaSO₄ powder was used as the 100%reflectance standard. The reflectance data were converted to absorptionaccording to the Kubelka-Munk equation al S=(1−R)²/2R, where R is thereflectance and a and S are the absorption and scattering coefficient,respectively. Thermogravimetric analysis was performed in a ShimadzuTGA-50 thermal analyzer by heating each sample (≈10 mg) from roomtemperature to 600° C. with a ramping rate of 5° C. min⁻¹ under nitrogenor air flow. TEM investigations were carried out in a JEOL 2100Ftransmission electron microscope operating at a 200 kV acceleratingvoltage. The sample was dispersed in ethanol and mounted on a carboncoated copper grid.

The copolymerization reaction of terephthalaldehyde with phloroglucinolwas drastically accelerated from the acid addition. The colorlessdioxane solution was rapidly transformed to orange gel 1 min after theaddition of some drops of the aqueous solution of HCl 1M. Thepolymerization was completed after heating the gels at 220° C. for 2days. At the molecular level the reaction is like that of microporousPOF1B (Katsoulidis, A. P.; Kanatzidis, M. G. Chem. Mater. 2011, 23,1818-1824), where terephthalaldehyde is transformed to a four-site nodelinking four phloroglucinols through methine carbons (Scheme 3) and eachphloroglucinol reacts with three aldehydes molecules

The formation of the polymer according to the above mentioned reactionwas proved with solid state ¹³C CPMAS NMR. A typical spectrum ofMac-POF-1 is presented in FIG. 14, where the side bands of the big peakare denoted with bullets. The resonance of the aldehyde carbonylcarbons, 195 ppm, did not appear and a new one emerged at 35 ppm whichis attributed to the methyne bridge carbons.

Porous properties of the Mac-POF-1 were investigated with N₂ adsorptiondesorption isotherms (FIG. 15). It exhibited type II isotherm, accordingto IUPAC, suggesting their macroporous texture, pore diameter >50 nm.The surface area of Mac-POF-1 was 1019 m² g⁻¹ and the total pore volumewas 1.13 cm³ g⁻¹. The high uptake of N₂ at low pressure indicated theexistence of micropores in the framework and the exact micropore volumewas calculated as 0.22 cm³ g⁻¹.

Mac-POF-1 was characterized with CW EPR spectroscopy at room temperature(FIG. 16). This compound was paramagnetic and showed strong EPR signals,which were 4 mT wide and centered around 341 mT (g factor=2.006). Thestabilization of unpaired electrons on the POFs was a simultaneouseffect of the polymerization reaction.

Silver Deposition:

The functionalization of Mac-POF-1 with Na⁺ ion-exchange prompted theinvestigation of some basic ion-exchange properties with metals such asAg⁺. The Mac-POF-1 had reductive properties and was capable of reducingAgNO₃, probably after ion-exchange as the silver ions enter thematerial. TEM images of Ag@Mac-POF-1 (FIG. 17) showed spherical Agnanoparticles of 5-10 nm in diameter well dispersed in the polymericframework. The XRD pattern of Ag@Mon-POF (FIG. 18A) shows broad peaksthat correspond to nanocrystalline cubic Ag (pdf 04-0783, ICDD) with anaverage crystallite size of ˜50 Å according to the Scherrer equation.The loading of Ag to Mac-POF-1 was significant at ˜20 wt. %, asestimated from thermogravimetric analysis. Ag@Mac-POF-1 exhibited an N₂adsorption-desorption isotherm (FIG. 18B) of type II as the pristinepolymer and a specific surface area of 608 m² g⁻¹, revealing that thedeposited Ag nanoparticles did not block the pores of the monolith.

The word “illustrative” is used herein to mean serving as an example,instance, or illustration. Any aspect or design described herein as“illustrative” is not necessarily to be construed as preferred oradvantageous over other aspects or designs. Further, for the purposes ofthis disclosure and unless otherwise specified, “a” or “an” means “oneor more”. Still further, the use of “and” or “or” is intended to include“and/or” unless specifically indicated otherwise.

The foregoing description of illustrative embodiments of the inventionhas been presented for purposes of illustration and of description. Itis not intended to be exhaustive or to limit the invention to theprecise form disclosed, and modifications and variations are possible inlight of the above teachings or may be acquired from practice of theinvention. The embodiments were chosen and described in order to explainthe principles of the invention and as practical applications of theinvention to enable one skilled in the art to utilize the invention invarious embodiments and with various modifications as suited to theparticular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto and theirequivalents.

What is claimed is:
 1. A composite material comprising: (a) a porous,three-dimensional, aromatic polymeric network comprising an organicpolymer comprising phenolic groups, wherein the phenolic groups arecrosslinked through linkages comprising one or more non-phenolicaromatic rings; and (b) metal nanoparticles distributed within thepolymeric network.
 2. The material of claim 1, wherein the phenolicgroups comprise hydroxynaphthalene groups.
 3. The material of claim 1,wherein the phenolic groups comprise phloroglucinol groups.
 4. Thematerial of claim 1, wherein the metal nanoparticles comprise silvernanoparticles.
 5. The material of claim 1, wherein the metalnanoparticles comprise lead nanoparticles.
 6. The material of claim 1,wherein the metal nanoparticles comprise gold nanoparticles.
 7. Thematerial of claim 1, wherein the metal nanoparticles comprise tin,bismuth or antimony nanoparticles.
 8. The material of claim 1, whereinthe polymeric network has a specific surface area of at least 500 m²/g,as measured by BET.
 9. The material of claim 1, wherein the polymericnetwork has stable unpaired electrons and exhibits a strong EPR signalat g≈2.006.
 10. The material of claim 1 having a metal nanoparticleloading of at least 20 percent by weight, based on the total weight ofthe organic polymer and the metal nanoparticles.
 11. The material ofclaim 1, wherein the phenolic groups comprise two or more aromaticrings.
 12. The material of claim 11, wherein the two or more aromaticrings are fused.
 13. The material of claim 1, wherein an aromatic ringof the phenolic group is functionalized with an —SH or an —NH₂ group.14. The material of claim 1, wherein the linkages comprise two or morenon-phenolic aromatic rings.
 15. The material of claim 1 having ahydroxyl functionality density in the range from 1 to 2 —OH groups perphenyl ring.